题目

Mark was walking home from school one day when he noticed a boy ahead of him had dropped all of the books he was carrying. Mark helped the boy pick them up.   16  they were going the same way, he carried them for the boy. As they walked, Mark knew the boy’s name was Bill. Mark   17  discovered that he loved computer games, baseball and history and that he was having a lot of   18  with his other subjects.They arrived at Bill’s home first and Mark was invited in   19  Coke and to watch some television. The afternoon passed   20  with a few laughs and some shared small ta如图(a)所示,质量为m=2kg的物块以初速度v0=20m/s从图中所示位置开始沿粗糙水平面向右运动,同时物块受到一水平向左的恒力F作用,在运动过程中物块速度随时间变化的规律如图(b)所示,g取10m/s2.试求:(1)物块在0-4s内的加速度a1的大小和4-8s内的加速度a2的大小;(2)恒力F的大小及物块与水平面间的动摩擦因数μ;(1)8s内恒力F所做的功.
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